The present invention relates to a method and an apparatus for forming thin-film layers of compound semiconductors of a I-III-VI.sub.2 system for use in solar cells and the like.
Compound semiconductors having a CIS (CuInSe.sub.2) thin-film layer have been used as solar cells and are known per se. FIGS. 4a to 4c show in section the principal steps for forming the CIS thin-film layer. A method of forming the conventional CIS thin-film layer will now be described with reference to these figures. First, a glass substrate 1 is overlaid with a chromium (Cr) layer 2 and a molybdenum (Mo) layer 3 that are formed successively by sputtering (FIG. 4a); then, a copper (Cu) layer 4 (about 2,000 .ANG.) and an elemental indium (In) layer or an In layer 5 (about 7,000 .ANG.) made of In containing selenium (Se) colloid (10 atm %) are superposed successively by electroplating; the assembly is heated in argon to form a precursor 4-5 for forming a Cu-In-Se thin-film layer (FIG. 4b); the assembly is further heated in a selenium (Se) vapor atmosphere to form a CIS thin-film layer 6 (FIG. 4c). The Cu layer 4 and the In layer 5 may alternatively be superposed by evaporation. The Se vapor is generated from a Se vapor source, or by heating a solid Se to evaporate it.
FIG. 7 shows in section the basic process of fabricating a solar cell from the conventional CIS thin-film layer. As shown, a CdS layer 7 is deposited on the CIS thin-film layer 6 by vacuum evaporation and, subsequently, a ZnO layer 8 is deposited on the CdS layer 7 by sputtering to thereby form a solar cell.
An apparatus (A) for forming the conventional CIS thin-film layer has been proposed in Japanese Patent Unexamined Publication No. Hei 8-107227. A closed space is formed between a lower heating jig and an upper heating jig, and the precursor and the solid Se (Se vapor source) are placed in the closed space and are heated to form a CIS-thin film layer in a Se vapor atmosphere.
Another apparatus (B) for forming the conventional CIS thin-film layer is shown in FIG. 5. This apparatus is basically composed of a reaction tube 11 and heating means 12 that is controlled by temperature control means 13. The reaction tube 11 has an Ar gas inlet 14 provided on top of a lateral side, with an exhaust port provided at the other end. Argon gas is introduced into the reaction tube 11 through a flow meter 15. The reaction tube 11 is also provided with a door 16 that can be opened such that a precursor 17 is brought into the tube and placed on a sample holder 18. The door 16 is fitted with a telescopic bar which in turn is provided with a vessel 19 at the distal end for holding a solid Se 20. The solid Se 20 is heated by the heating means 12 to form a gas atmosphere consisting of a mixture of the Ar gas and the Se vapor. In this mixed gas atmosphere, Se is introduced into the precursor to form a mixed crystal and, hence, the intended CIS thin-film layer. The exhaust gas is discharged from the exhaust port and subjected to a waste treatment.
Yet another apparatus (C) for forming the conventional CIS thin-film layer is shown in FIG. 6. This apparatus is basically composed of a closed ampule 11' in the form of a quartz tube and heating means 12, and its principal use is in the laboratory. The solid Se 20 is heated by the heating means 12 to form a Se vapor atmosphere, in which Se is introduced into a layered structure 17 having a precursor to form a mixed crystal and, hence, the intended CIS thin-film layer.
In the conventional apparatus (A) for forming the CIS thin-film layer, both the precursor and the solid Se are heated as they are placed within the closed space maintained under vacuum and, hence, the desired selenization reaction can be adequately accelerated within the generated high-pressure Se vapor atmosphere, on the other hand, the precursor cannot be selenized and sulfurized either simultaneously or separately within the same vacuum atmosphere.
The apparatus (B) for forming the conventional CIS thin-film layer has had the following problems: i) after the selenization reaction, a large amount of the unreacted Se has to be discarded; ii) as a result, a large amount of solid Se has to be employed; iii) vacuum cannot be created in the annealing furnace and, hence, if the precursor is prepared by a wet process such as electroplating, the moisture which affects the quality of CIS cannot be thoroughly removed before the selenization reaction.
The apparatus (C) for forming the conventional CIS thin-film layer employs a small ampule as the reaction vessel and, hence, it is suitable for small-scale operation in the laboratory but not adapted to high-volume production on an industrial scale.